As shown in FIG. 1, the ordinary shadow mask type color picture tube is constituted such that a luminescent screen 12 is formed on the inner surface of the face plate of a panel 11, a shadow mask 13 supported by a frame 14 is installed in such a manner as to be separated as much as a certain gap from the luminescent screen 12, and electron beams emitted from an electron gun (not shown) are made to selectively land on the luminescent screen 12 by means of the shadow mask 13 so that the intended image should be formed by visible light emitting from of the luminescent screen 12.
The electron beams emitted by the electron gun are deflected by deflecting means such as deflecting coils and the like, so that the beams should land on the visible region of the luminescent screen 12. However, the so-called over-scanned beams which are produced by departing from the effective region after being excessively deflected are reflected from the side face of the panel or the side end of an opening portion of the frame, resulting in that a twice (i.e., double) luminescence is caused, thereby degrading the image quality through the aggravations of the luminance and purity, and producing the so-called halation phenomenon, in other words "visible electron flooding".
As a measure against this phenomenon, an electron shield 15 made of a thin metal sheet is installed in the space between the side face of the panel 11 and the outer face of the frame 14 as a blocking element against the reflections from the side face of the panel, while one of various kinds of elements are provided against the reflections from the side end of the opening portion of the frame 14.
To describe in a more detail the reflection mechanism of the beams from the side end of the open portion of the frame 14, this frame 14 has usually a thickness of 1 to 2 mm, and therefore, ineffective electron beams of the outermost region of the effective region i.e., the ineffectively overscanned electron beams having a deflection angle that is a little larger than the maximum deflection angle will be incoming to the side end of the open portion, while the reflected electron beams (shown by the dotted lines in FIG. 1) are scanned onto the luminescent screen 12, thereby producing the halations.
The extensively used conventional method for preventing such a phenomenon is constituted such that, instead of the frame, the electron shield 15 is extended up to the inside of the open portion of the frame 14, thereby limiting the outer boundary of the effective region. In such a case, the leading end of the electron shield 15 having a thickness (usually 0.2 mm or less) far thinner than that of the frame 14, has a small reflection area, and therefore, it can effectively prevent halation. However, in order to limit the outer boundary of the effective region by means of the electron shield 15 having so thin a thickness, an extremely high degree of precision is required in its fabrication and assembly, as well as making the workability very fastidious, thereby making it difficult to expect a saving of manufacturing cost.
Under this circumstance, various techniques have been proposed, attempting to limit the outer boundary of the effective region by means of a shadow mask frame, and to modify the shape of the leading end of the open portion of the frame, in order to prevent halations. However, almost all of them were of the technical constitution such that the reflection area for the ineffective electron beams at the side end of the open portion of the frame is to be reduced.
For example, one of these techniques is disclosed in Japanese Patent Publication No. 58-9539, and is illustrated in FIG. 2. This technique is constituted such that, in the ordinary shadow mask type color picture tube described above referring to FIG. 1, the edge of the open portion of the frame 14' of the shadow mask 13 is bent toward the electron gun which is the electron beam source, and the leading end of the bent portion is provided with a sharp burr 19 in order to minimize the reflection area for the ineffective electron beams.
However, the method of providing the burr 19 to the leading end of the open portion of the frame 14' is constituted such that the burrs are produced between a punch and die when carrying out the blanking process, and therefore, the shapes of the burrs are not uniform, with the result that not only the halation prevention effect is not sufficient, but also there are many problems in handling the component because the burr forms a sharp edge.